1. Field of the Invention
This invention relates generally to devices for measuring the flow rate of a fluid moving through a conduit, and more particularly to a flowmeter of the type which disturbs the flow stream for producing modulated sub-flows and includes a sensor for responding to the electrical activity generated by the sub-flows.
2. Description of the Prior Art
It is known that a fluid moving in a conduit against around various obstructions such as valves and bends of elbows generates electrical activity within the fluid. In U.S. Pat. No. 4,363,244 issued to Rabeh and Hemp a flowmeter is disclosed which is sensitive to such electrical activity. Rabeh and Hemp teach that this electrical activity is believed to be the result of electrical charges in the fluid. Therefore, as the fluid containing these electrical charges moves through the conduit, a noise signal is produced which can be detected by a series of electrodes disposed along the fluid flow stream. One aspect of the invention disclosed in the '244 patent is that the mean frequency of the received noise signal is taught to be proportional to the flow rate of the fluid.
However, the electrical charges are thought to be irregularly distributed in the fluid so that the received signal includes many randomly occurring peaks and is not a well-defined, smoothly changing signal that is easily detected and processed. Statistical methods are thus necessary for calculating the fluid flow rate. Since the peaks of the received signal are widely scattered over time, the accuracy of any short term calculation of the flow rate will be degraded. Accordingly, it appears that the patent discloses a relatively slow reacting device.
Moreover, the '244 patent teaches the use of circuits which square, integrate, divide and determine the square root of various parts of the received noise signal. Though such circuits are well known, they become more complex and costly as greater precision is required.
U.S. Pat. No. 3,813,939 issued to Head discloses another flowmeter responsive to electrical charges contained in a moving fluid. The Head patent teaches that a pair of electrodes disposed in a pipe are usable for sensing the field signal generated by the movement of the fluid containing the charges. Being separated by a known distance, the two electrodes produce signals which are amplified and algebraically combined with one another to yield a difference signal. By applying this difference signal to an auto-correlator, the transit time of the electrical charges can be determined and the fluid flow rate can be thus calculated therefrom. However, it is admitted in the '939 patent that the invention described therein is suitable only for applications where relatively low accuracy and slow response are acceptable, such as where an alarm or safety valve is actuated in response to a gross change in flow rate. In other words, since the flowmeter disclosed by Head is still based on a statistical procedure, it suffers from the same drawbacks as previously mentioned for the device mentioned in the '244 patent.
As a result of the above, the flowmeters disclosed in the '244 and '939 patents are generally not suitable for industrial process applications. Therefore, there is a need for an improved flowmeter which performs relatively quickly and accurately when responding to the electrical activity produced by fluid moving through a pipe.